1. Field of the Invention
The present invention relates to a driving method of a plasma display panel (PDP).
A PDP is commercialized as a wall-hung television or a monitor of a computer. A PDP is a digital display device having binary light emission cells and is suitable for displaying digital data, which is expected to be used as a multimedia monitor. One of problems to be solved for a PDP is to reduce background luminance.
2. Description of the Prior Art
In an AC type PDP for color display, a three-electrode surface discharge structure is adopted. In this structure, display electrodes to be anodes and cathodes for display discharges are arranged in parallel on the inner side of one of the substrates, and address electrodes are arranged so as to cross the display electrode pairs. Three electrodes work for a cell that is a light emission element unit. In the surface discharge structure, three types of fluorescent material layers for color display are arranged on a second substrate that faces to a first substrate on which the display electrode pairs are arranged, so that deterioration of the fluorescent material layers due to an ion shock upon discharge can be reduced and long life can be obtained. In general, the address electrodes are also arranged on the second substrate and are covered with the fluorescent material layers.
In the PDP display of the surface discharge type, one of the display electrode pair corresponding to a row is used as a scan electrode for row selection. Between the scan electrode and the address electrode, an address discharge is generated, which causes an address discharge between display electrodes, so as to control a charge quantity in a dielectric layer (a wall charge quantity) as addressing. Then, display discharges are generated plural times corresponding to the display luminance as sustaining by using the wall charge. Further, a process (reset) of equalizing an electrification state of the entire screen is performed prior to the addressing. When the sustaining finishes, there are cells with remaining relatively much wall charge and cells with remaining little wall charge. Therefore, the reset process is performed as an addressing preparation process for enhancing reliability of the display.
In the U.S. Pat. No. 5,745,086, the reset process is disclosed, in which a first ramp voltage and a second ramp voltage are applied to cells sequentially. When applying the ramp voltage having a small gradient, in accordance with characteristics of a micro discharge that will be explained below, light quantity of a light emission in the reset period is decreased for preventing a contrast drop, and the wall voltage can be set to any target value regardless of variation of the cell structure.
When a ramp voltage with increasing amplitude is applied to a cell having an appropriate quantity of wall charge, plural micro discharges occur while the applied voltage increases if the ramp voltage has a small gradient. If the gradient is smaller than this, a continuous discharge occurs with short discharge period. In the following explanation, both the periodical discharge and the continuous discharge are called xe2x80x9cmicro dischargexe2x80x9d. In the period generating the micro discharge, even if a cell voltage (=wall voltage+applied voltage) exceeds a discharge start threshold level due to increase of the ramp voltage, the cell voltage is always kept at the vicinity of the discharge start threshold level. It is because that the micro discharge drops the wall voltage by equivalent to the increase of the ramp voltage. Since the discharge start threshold level is a constant value determined by electric characteristics of a cell, the wall voltage can be set to any value that is suitable for the addressing by setting the final value of the ramp voltage. Namely, even if there is a minute difference of the discharge start threshold level between cells, a relative difference between the discharge start threshold level and the wall voltage can be equalized in all cells.
In the reset process utilizing the characteristics of the micro discharge, the first ramp voltage is applied so as to form an appropriate quantity of wall charge in the cell, and then the second ramp voltage is applied so that the wall voltage between the electrodes becomes close to the target value. The amplitude of the first ramp voltage is set so that the micro discharge is always generated by the second ramp voltage. In addition, the polarity of the second ramp voltage is set to be the same as that of the voltage that is applied in addressing.
Conventionally, control of the electrode potential in the reset process is uniform in all cells.
However, it was a problem in the reset by the conventional driving method that reduction of a background light emission is difficult. The background light emission is a light emission in an area of the screen that is not to be lighted. Another problem is that the background light emission can gain a color, resulting in a deterioration of gradations in color. Causes of these problems will be described below.
FIG. 34A shows three voltage waveforms (the applied voltage, the wall voltage and the cell voltage) between YA electrodes in the conventional reset process. FIG. 34B shows a transition of an integral light emission quantity in a reset period TR. The language xe2x80x9cbetween YA electrodesxe2x80x9d means between the scan electrode and the address electrode, and the language xe2x80x9cintegral light emission quantityxe2x80x9d means a sum of the light emission quantity in the case where an optional period is paid attention. In the example shown in FIGS. 34A and 34B, the wall voltage just before the reset process is a constant value regardless of the fluorescent material. Characteristics of red, green and blue fluorescent materials are indicated with a broken line, a full line and a chain line, respectively.
Three types (red, green and blue) of fluorescent materials are used for color display. Usually, these fluorescent materials have different properties, particle diameters and surface states of layers. This means that the discharge characteristics of the cell can be affected not only by the variation of the cell structure due to a production process but also by difference in type of the fluorescent material. The difference of the discharge start threshold level between cells of different fluorescent material types can be 50 volts or more.
Here, the case where the discharge start threshold level between YA electrodes is unique to each light emission color of the fluorescent material will be explained. When the address electrodes are the cathodes, the discharge start threshold levels of red, green and blue colors between YA electrodes are denoted as VtYA(R), VtYA(G) and VtYA(B). It is supposed that the following relationship is satisfied.
VtYA(R) less than VtYA(B) less than VtYA(G)xe2x80x83xe2x80x83(1) 
Then, as shown in FIG. 34A, discharges are generated in different time points for each light emission color. When the address electrodes are the anodes, the discharge start threshold level VtAY between YA electrodes is regarded as a constant value regardless of the fluorescent material. Since the discharge start threshold level depends mainly on a secondary electron emission coefficient of dielectric in the cathode side, the above assumption is practical. However, this argument can be easily applied also to the case where the discharge start threshold level VtAY depends on the fluorescent material.
When the first ramp voltage (a write pulse) is applied, the micro discharge starts in the order of red, blue and green in accordance with the relationship (1). Therefore, the light emission period is the longest in red cells, second longest in blue cells, and the shortest in green cells. In addition, the variations of the wall charge in red, green and blue cells are different from each other, so the wall voltage values are different between red, green and blue cells when the application of the first ramp voltage finishes. Therefore, the micro discharge starts in the order of red, blue and green colors also when the second ramp voltage (a compensating discharge pulse) is applied, so that the light emission period is longer in the order of red, blue and green.
The amplitudes V1YA and V2YA of the ramp waveform are set so that a discharge is generated securely in green cells, which are hardest to generate a discharge among three colors. Therefore, light emission quantities of red and blue colors are naturally larger than that of green color, so that luminance of the background light emission increases. Furthermore, since a valance among red, green and blue colors is lost, the background light emission color is not a white color with small luminosity (a dark gray color) but a reddish color. It can be a bluish color depending on a selection of the fluorescent material.
An object of the present invention is to reduce the background light emission so that contrast of display can be improved.
In the present invention, address electrodes are grouped in accordance with discharge characteristics of cells corresponding to each of the address electrode, and potential control is performed, which is unique to each group so that luminance of the discharge light emission in a reset becomes uniform among cells having different discharge characteristics in the reset that is preparation for addressing. In other words, discharge intensities and light emission periods of other cells are optimized so that the luminance is adapted to that of the cell having the lowest luminance, by controlling potential for each group.
A typical example of grouping is to group in accordance with a type of the fluorescent material. If the discharge characteristics are different among three cells having different fluorescent materials, the address electrodes are divided into three groups. If one type is different from the other two types concerning the discharge characteristics, the address electrodes are divided into two groups. If the discharge characteristics are different depending on a position in the screen, two or more groups are made.